X-ray spectrometer



Feb. 6, 1951 HARKER 2,540,821

X-RAY SPECTROMETER Filed April 19, 1949 2 Sheets-Sheet l wewme: amexs'mzPas/170 F Inventor":

1 David l-lar- Ker;

/ by m M His Attorney.

Feb. 6, 1951 D. HARKER 2,540,821

' X-RAY SPECTROMETER Filed April 19, 1949 2 Sheets-Sheet 2 A ra'meInvent-or": David Hawker,

His Attohney.

Patented F ch. 6, 1951 X-RAY SPECTROMETER David Harker,- Schenectady,N..Y., assignonto General Electric Company, a. corporation of New YorkApplication April 19, 1949, Saris/1N0. 88,461

15 Claims. .1

This invention. relates generally to analysis by the use of X-rays.

In the study of. materials and their properties, it is well known thatagiven material may be irradiated by primary rays or charged particlessuch that it will emit a fluorescent radiation. This fluorescentradiation is characteristic of the atomic composition of the material inthat it is composed of X-rays having certain wavelengths and intensityratios which arevalways the same for a given material irradiated by agiven source. A plot of intensity versus wavelength of fluorescent.X-rays is commonly known as the fluorescentX-ray spectrum of thematerial and may be used advantageously as a means for identifying oranalyzing the material.

A principal problem; in utilizing the. abovedescribed properties ofX-rays lies in the, d-ifliculty oi obtaining fluorescent radiations ofsufficient intensity to permit facile detection. Much of the intensity'of the irradiating or primary beam is expended in theflphenomena ofsca-tter-ing and, eventhough a relatively intense primary beam isemployed, the fluorescent radiation does not attain an intensity whichis readily observed. Furthermore, since the intensities of the componentwave-lengthsmust be individually measured to obtain a fluorescent X-rayspectrum. additional intensity depleting factors may be introduced bythe means adopted .for separating the spectrum intoits components. 7

.It has been suggested heretofore that the apparent intensity of thefluorescent X-rays may be increased by dilfraction with a bent crystal.By this expedient X-rays maybe monochromatized, i. e. those of a givensingle wave-length separated from the heterogeneous fluorescentradiation, and, at the same time,,'may be focused at a relativelydefined spot. A detector may then be located at this spot to secure theadvantage ofthe increased intensity.

In order to observe the X-ray spectrum of a material, however, it isnecessary to measure separately each X-ray wave-length within thefluorescent radiation. The particular wavelength which is focused anddetected .at one time depends upon the relative positions of thematerial under study and the detector, as well as the position andcurvature of the bent crystal. Heretofore; the spectrum has beenobtained by employing a crystalhaving a fixed curvature and moving thecrystal and the detector with respect to themater'ial land to each othersuch that the requisite focusing relations are maintained for each ofthe wave-lengths to be measured. This necessarily resultsin the detectorapproaching the crystal as the spectrumis traversed, with theconsequencesthat the focusing becomes less-eiiective and-scatteredprimary radiation gives spurious signals to the detector. Ac-

cordingly,..i=t. is a principal object of this inven-:

tion to. provide an improved, mechanically; advantageous bent crystalX-ray spectrometer which is capable of realizing a high degree of tralaxis of the flexible crystal.

The aspects of, the invention whichltis desired to protect herein arepointed out with particularityin the. appended claims. The inventionitself, together. with further objects and advantages, may best beunderstood by references tothe' following description taken inconnection with the drawings in which. Fig. '.'1 is a diagram-i maticrepresentation useful in explaining the principle of the invention, Fig..2-is a partially exploded. diagrammatic view suitably embodying theinvention andlEigs, 3 and 4 are diagrammatic illustrations illustratingOperational characteristics of the invention.

To obtain a better understanding of the principles and features of;theinvention reference may be had to Fig. -1 wherein .a specimen S maybe consideredasbeingirradiated by asource of polychromatic, .hardX-rayssuch that its characteristic fluorescent X-rayyspectrum will be emitted.Some of the rays in this spectrum mayibesrepresented as indicated .by..'1ines A1,;A2, and .As and will pass through a-thin crystal Accordingtotheoretical concepts, X-rays are diffracted by a crystal in such a waythat they may be considered tabs-reflected from planes of atoms existingin the crystal. Such planes of atoms are often called lattice planes.The .relation according to 'which this diffraction occursthe so-calledBragg '1aw-.is most conveniently formulated in terms of this conceptualreflection. In these terms the Braggiawis:

m=2d sin 6 (1) where d is the distance between lattice planes, 0 is theangle oidnoidenoe or emergence of the X-rays. his the wavelength of theX-rays and 'n is a whole-.numbencalledthe order.

. From this relation rit may be :seen that ii crystal M has acertaininterplanar spacing d andis placed towintersect lninthe path ofthe fluorescent rays emanating from S. such that asheaf of its latticeplanesv perpendicular to the, plane of the zdrawing form an angle :0with respect to the central incident ray A2103? wavelen th A, in-

tel-sect circle C1 at point :1: along a path forming at point y oncircle 01 will also be diffracted to intersect circle C1 at point x eventhough they would have formed incident angles other than with crystal Mhad it remained flat. :Thus, it U may be seen that rays in thefluorescent spectrum of S having a single wavelength i may 1 be focussedat point a: on circle-C1 which has its center at P1. A detector D ofX-rays having a relatively narrow entrance aperturemay then be H locatedat point r to receive the rays. In practical use, if the interplanarspacing d of crystal M is known and the position of detector D ismeasured,.t'he wavelengthskmay be caluculated. J

Amer the rays of wavelength A have. been measured, rays within thefluorescent X-ray spectrum S having other frequencies. or wavelengthsmay be measured by suitably altering the spatial interrelationship ofthe crystal M and detector D. It may be shown that if the radiusoicurvature of crystal M is left fixed while the crystal is rotated aboutan axis through P1, the various wavelengths within the fluorescentspectrum of S .will be .focussed successively at points spatiallydistributed along the circumference of V circle C2 which has a radius ofR2 and a center at point P2. Thecircumference of C2 is also tangent .tocrystal M at P1. It will be apparent,.then,.that if detector D is movedalong C2 at such a rate that the line Pix rotates at twice theangular.speed with which crystal M is rotated, each wavelength within thefluorescent spectrum of S. may be detected individually andsuccessively. Thus, froni the -intensity measurements at thevariousangular positions of D, the X-ray. spectrum of S may be obtained.

l As has been heretofore mentioned, however, this arrangement hascertain'disadvantages resulting. from the necessity of moving detector Dalong Cz'to receive the focussed rays. According to the;present-invention, the radius of curvature of crystal M is made variableto permit deas it is rotated. Thus, it may be observed that as the angle0 is varied, by rotating crystal M, to

obtain the fluorescent X-ray spectrum of S, the

desired focussing relations may be maintained even though detector Dismoved along circle C1.

In Fig. 2 there is shown an exemplary illustration of apparatus suitablyadapted to the realization of the above described principles andfeatures A source l, which may be a generator of hard, polychrornaticX-rays, is positioned to irradiate a' specimen 2 which has asubstantially flat exterior iace portion 3 placed at a suitable.

angle, such as 45, with respect to the incident rays from source lrepresented by broken line :3. As has been heretofore-explained; theirradiation of specimen 2 will excite fluorescent X-rays characteristicof its atomic composition. Some of such fluorescent X-rays will beemitted approximately perpendicular to the incident rays 4 as indicatedby broken line 5 and will pass through a baiiie 5 which serves toexclude stray manner to be more fully described hereinafter.

; holder tector D tobemoved along a circle C1 ofradius Ri to receive thefocussed rays rather than along circle '02, :thus 1 maintaining aconstant distance between detector D and the axis of rotation-0f crystalM. I

.. From .the diagram o f Fig. 1 it may be shown c where 5 is theperpendicular distance between the tangent to'the crystal at axis P1 anda line parallel to the tangent intersecting the crystal contact pointsof members F and G. From this relation it is apparent that bymaintaining L a constant and suitably varying 6, the desired focussingof the various X-rays may be secured by placing detector D on C1. Also,if L and R1 are constants,- 5 becomes a function of 0, therebypermitting the proper bending of crystal M to be directly determined bythe position of the crystal For the purpose. of rotating and varying thecurvature of crystal 8, which may consist of a thin, rectangular cut ofcrystalline material such as mica or quartz, there is provided a crystalI2 which comprises a generally rectangular base plate i3 rigidlyattached to a rotatable axle I l. Crystal 8 is supported. on a slidableplate [5 between two upright right angle contact members It and ll whichmay be formed integrally with plate l5. The center portion of crystal 8bears against the apices it, which lie on the axis of axle it, oftriangularly shaped members [8 that are attached to or integral with ahollow, box-shaped member Q whichis more clearly illustrated in Fig. 3.Members l8 and Lil may be securely fastened to base plate 13 by means ofscrews l9. Right angle bearing members 28, one of which is not shown,serve to hold slidable plate l5 against the upper surface of base plateI 3 while permitting transverse movement. Depending from a protrudinglip portion 2| of slidable plate i5 is an axle 22, to the'lo-wer end ofwhichis attached aroller 23 that bears against the face 2 1 of astationary cam 25. As will now appear, when axle it is turned about itsfixed longitudinal axis, crystal holder [2 will turn in synchronismtherewith while the ends of crystal 8 will move back and forth withrespect to base plate 13 in response to the position of slidable plateIt which is determined by the cam 25; Thus, since the center portion ofcrystal 8 is maintained at a fixed position with respect to base platel3 by means of triangular shaped ture as axle I4 is rotated. In order tosatisfy the conditions of Equation 3, cam 25 must be suitably weasel itlllfi i a I hereinafter,

To .provide a means, for; rotating detector l l men-1 :mannerisms-1.1

in synchrcnism with crystal holder suchithat h X- y s tru i ec es ma basitx ably scanned, there. isshown alradi u s arm zfi affixed to, an axle2? which isladapted-tobedriven in conventional fashion such as bytogthed plate V an sas sworm g ar 14-; hel nsitudir. nal axisoi,ax1e..2l is collinear with the longitudinal axis ,oiaxle, l 4 anddectector l is mount:

ed. o d s a 2 as ai1i sn ance slitl ifiwill be upon an imaginary. circlehaving a centeron theaxis of axle, as hown in Fig'. ;4.,

Axledd is driven athalf the; an axle 2? by the motion cated generally bynumeral 29 it comprises an arm 33 tached' at one end toiaiprotrudingilip3| ofradius arm 26 by; means oiamivet orbolt 32.

translating means indi-g L shermmems: iWh1uh lS rotaialoly ate Rotatablysecuredat thenother -eendi-of arm i by means oi an upright post 1133 isone endof an arm 34, the remainingend of which is irotatably attached tofixed support 35. Upright'post 33 is slidably positioned withintheislotted. portion on an arm 33 which is rigidly fixedoto rotatableaxle M, This exemplary lmotionitranslating means will serve to rotate;axle: Mu at halfthe speed of axle 21 as wormZS-(uisrotated, providingthe lengths oi arms 3o..and34 areequaL and the distances from the axisof axle (21: totheaxes of post 35 andbo1t132. are equal. In such-event,arm 36 will always bisect-opposite angles of the A imaginaryquadrilateral; 1 thereby- -assuring the i desired relationship.

Indication of the angular. position of--.detector means of a calibratedscale'efiwforrned integrally with radius arm-"-26, and a fixedpointer3'1: Scale Biifihas a shape conforming tothearc H. and crystal holdersmay. be-iobtained by of a circle concentric.withthecircular pathfollowed by the entrance slit 1 i of detector H as worm. 28' is turnedto obtain the-spectrum of specimenZ.

In order to. obtaina permanent record of the spectrum orT-specimen2,-there is provided a recorder 3 i :oi a conventionaltype-having itsinput connected to the output of detector l [by means I of leads 39.Signals from detector 1 I, generated by X-rays from specimen 2, aretransmitted to the recorder element 40 and recorded as intensities ofX-radiation upon chart 4| by meanswof pen 42. The trace recorded bypen421may be made a function of the positionof detector II and crystal '8by moving; the chartwith a con-,

Crystal 8 is with respect to centralray path 5. (Az in Fig. -1).

while entrance sliti I of detector ll (D inFig,

1) is positioned to admit thedifiracted rays 9 at an angle20. 'Ihus,providing-the.interplanar spacings d of crystal 8 (Min Fig are oi-theproper, value, Equationl willbesatisfied forray s h ri ie. a w vel n thds-d tector i W lis-t 75,

dicatei t e t ns i t eyi o loiroa hsfi amt-8.1. Inaddition, rays ofwavelength r emanating from specimen- 2 which have incident angles;other than 0 will be diiiracted and iocussed at;

point ac coincident with entrance slit ll, provid ing the curvature R3of crystal 8 islsuch as to H satisfy Equation 2. This requirement maybe1 fulfilled by suitable location; of stationary cam;-

25 as will appear subsequently upon radius arm 26. Detector II willfollow the circular path or focussing circle C1 and its relationshipwith the rotation of crystal 8 in, holder Q will always be such as tosatisfy Equationl for various values of 0, since detector ll moves insynchronism with and at twice the angular speed of'axle 14. Moreover, asholder 2 rotates on axle l4, roller 23 will move synchronously therewithalong face 24 of stationary cam 25,

thereby causing slidable plate l5 to move transversely in responsethereto within bearing mem-t bers 20.

cident with the axis of axle I4, by means of contact members 16 and I!that are attached to slidable plate I 5. It may be shown from Equation 3that the proper deflection 6 of the contact members H5 and IT, asmeasured from their position. when crystal 8 is permitted to lie flat orin the position of maximum radius of curvature, may be secured if theface 24 of stationary cam 25 conforms to the arc of a circle of aconvenient radius whose center is a distance L /2R2 from i the axis ofaxle [4 (P1 in Fig. 1) along a line joining the axis of axle l4 andpoint y. L represents the distance of each contact member l6 and ll fromthe axis'of axle l4 while R1 is the radius of focussing circle C1,'thepath followed by detector H. In this mannerpthe proper focussingrelationships may be maintained as screw 28' is rotated.

In the operation of the invention, screw 28 is turned to move the radiusarm 26 and detector- [I through the range from 20=0 (see Fig. 5) atwhich maximum curvature of crystal 8 occurs,-

toward 2c=180 as indicated by arrow 48adjacent calibrated scale 36.(Fig. 2). For practical purposes, 26 usually need not exceed v which isthe maximum value shown on scale 36' A given wavelength within thefluorescent spectrum of specimen 2 will be indicated as a peak on chart4i whenever the incident angle 0 satisfies Equation 1 for successiveinteger values of 'n, e. g. when Another given wavelength will beindicated at a correspondingly different series of values-for 0. Thus,the complete fluorescent spectrum of a specimen may be obtained.

As an illustration of a practical adaptation of this invention specimen2 may be considered as consisting of pure copper having a characteristicspectrum comprising wavelengths:

Ka2=1.5412kX (The symbol kX represents-kilo-X-units of wave lengthsLengths in kilo-X-units may be trans, formed into lengths in centimetersby multiply-. l ingby LOOZOZX 10-?) l If flexible ;crysta1.= 8 con-r I.

As screw 28" (Fig. 2) is turned,,crystal holder. v l2nwill rotate onaxle I4 anddetector l I will move--v Accordingly, crystal 8 willbebentto; present a convex surface to the incident fluoresi centradiation about apices I8,.which are coinsists of mica having a sheaf oflattice planes perdicular to its cleavage with spacings do2o=4.517cX,

then peaks will be indicated on chart 45 at values of 20 varying fromapproximately 20=17 41,

for n=1 with wavelength Kc, to 29=1l7 22, for r 71 with wavelength Kaz.Since each element has a different characteristic spectrum containingcomponent wavelengths of this nature, specimen 2 may consist of anunknown material which can be analyzed for the elements it contains,qualitatively from the values of 20 (or wavelengths) at which peaksoccur and quantitatively from the intensities indicated by the heightsof the peaks.

Although the description has been concerned mainly with one embodimentof the motion translating means 23, it is obvious that other means, suchas gears, may be equivalently emplcyed, providing backlash tolerancesnecessary for the desired accuracy are not exceeded. Moreover, sincesimilar motion translating means, as well as similar X-ray detecting andrecording means, may be utilized in X-ray diffraction goniometerapparatus, it will be apparent that the instant invention will finduseful application in providing conjunctive analytical apparatus.

It will also be observed that the specimen 2,

illustrated in Fig. 2, may be any material in solid, liquid or gaseousform and need not be located at any particular angle with respect to theincident rays from source l, providing the excited fluorescent radiationmay be viewed by crystal 8;

In some instances it may be desirable to insert soller slits in the pathof the fluorescent radiation immediately before and after its passagethrough crystal 8 in order to improve the fineness of focussing.Furthermore, it will be noted that the source I need not be restrictedto a source of hard X-rays. Any means of exciting the fluorescent X-rayspectrum of a specimen being analyzed may be employed with efficacy; e.g. bombardment with electrons or other charged particles having asuitable energy. Since some radioactive materials excite their ownfluorescent X-ray spectra, no external excitation is necessary whenspecimens of such materials are being analyzed.

It will be understood, therefore, that while the invention has beendescribed by reference to particular embodiments thereof, numerouschanges may be made without actually departing from the invention, andin the appended claims it is intended to cover such equivalentvariations of application and structure as are within the true spiritand scope of the foregoing disclosure.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. The method of obtaining a fluorescent X-' 2. The method of measuringthe componentwavelengths within the characteristic fluorescent X-rayspectrum of a material which comprises irradiating a specimen to exciteits fluorescent radiation, positioning av flexible crystal withinthepath of said radiation to difiract said radiation, rotatingandsimultaneously' bending said crystalabout a fixed axis to focus saiddiffracted radiation at spatially distributed points corresponding torespective wavelengths alongja focussing circle having its center onsaid axis, and moving a detectoralong said focussing circle insynchronism with the rotation of said crystal to receive the diffractedradiation.

3. The method of measuring the component 7 wavelengths within thecharacteristic fluorescent X-ray spectrum of a material which comprisesrotating a flexible crystal within the path of the fluorescent radiationto monochromatize successively the component wavelengths of saidradiation as a function of the angular position of said crystal withrespect to' central incident radiation, variably bending said crystalabout its axis of rotation in conjunction with its rotation to focussuccessively said components along a focussing circle having its centeron said axis, and moving a detector along said focussing circle inrelation to the rotation of said crystal to receive said focussedcomponents.

5. In a method of measuring the component Wavelengths within thecharacteristic X-ray spectrum of a material by rotating a crystal Withinthe fluorescent radiation to monochromatize successively the variouscomponents along spatially distributed paths intersecting a focussingcircle, the improvement which comprises bending said crystal about itsaxis of rotation in synchronism therewith whereby said components may befocussed successively along a focussing circle having its center on theaxis of rotation of said crystal.

6. In apparatus for measuring the component 'wavelengths of radiationwithin the fluorescent X-ray spectrum of a material, a flexible crystalmounted rotatably about an axis in the path of the fluorescentX-radiation to diffract component wavelengths selectively as a functionof its angular position, means for detecting the diffracted radiationmounted movablyas a function of the angular position of said crystalalong a circular path having an axis collinear with the axis of rotationof said crystal, and means for bending said crystal to conform to thearc of a circle intersecting the axis of rotation of said crystal andhaving a radius of curvature variable as a function of the angularposition of said crystal.

7. In apparatus for measuring the component wavelengths of radiationwithin the fluorescent X-ray spectrum of a material, a flexible crystalmounted rotatably about an axis in the path of the fluorescentX-radiation to diifract component wavelengths selectively as a functionof its angular position, means for detecting the diffracted radiationmounted movably along .a circular path having an axis collinear with theaxis of rotation of said crystalbmotiontranslat- --ing means forrotating said'crystal and moving said detecting means in synchronism,and means for bending said crystal to conform to the arc of a circleintersecting'the i axis of rotation of 8. In apparatus for measuringthe" component wavelengths of radiation within the fluorescent X-rayspectrum of a material, a flexible crystal mounted movably about an axisin the path of the fluorescent X-radiation to monochromatize thecomponent wavelengths along spatially distributed paths intersecting afocussing circle which has its center on said axis, means movable alongsaid focussing circle in synchronism with the rotation of said crystalfor detecting said component wavelengths, and means for bending saidcrystal to conform to the arc of a circle intersecting the axis ofrotation of said crystal and having a radius of curvature variable insynchronism with the rotation of said crystal whereby each of saidcomponent wavelengths of radiation may be focussed successively alongsaid focussing circle for detection.

9. In apparatus for measuring the component wavelengths of radiationwithin the fluorescent X-ray spectrum of a material, a flexible crystalsupported rotatably about an axis substantially parallel to a sheaf ofits lattice planes in the path of the fluorescent X-radiation tomonochromatize the component wavelengths along spatially distributedpaths intersecting a focussing circle which has its center on said axis,means movable along said focussing circle in synchronism with therotation of said crystal for detecting said component wavelengths, andcam operated means for bending said crystal to conform to the arc of acircle intersecting the axis of rotation of said crystal and having aradius of curvature variable in synchronism with the rotation of saidcrystal whereby each of the component wavelengths may be focussedsuccessively along said focussing circle for detection.

10. In apparatus for measuring the component wavelengths of radiationwithin the fluorescent X-ray spectrum of a material, a flexible crystalsupported rotatably about an axis substantially parallel to a sheaf ofits lattice planes in the path of the fluorescent X-radiation tomonochromatize the component wavelengths along spatially distributedpaths intersecting a focussing circle which has its center on said axis,cam operated means for bending said crystal about its aforementionedaxis to conform to the shape of the arc of a circle, detecting meansmovable along said focussing circle for receiving said componentwavelengths, and motion tran lating means for rotating said crystal,moving said detecting means and actuating said cam operated means insynchronism whereby each of the component wavelengths may be focussedsuccessively along said focussing circle for detection.

11. In apparatus for measuring the component wavelengths of radiationwithin the fluorescent X-ray spectrum of a material, a flexible crystalhaving a central portion and end portions, said crystal being rotatablysupported about an axis traversing said central portion and in the pathof the radiation to monochromatize the component wavelengths alongspatially distributed paths intersecting a focussing circle; movablecontact members bearing against the end portions of said crystal forbending said crystal about said axis bers-and; moving said detecting topresent a generally convex surface to said radiation; detecting meansmovable alongsaid focus'sing'"circle for receiving said componentwavelengths; and motion translating means for rotating saidbrystal,moving said contact memwhereby each of the component wavelengths may, befocussedsuccessively along said iocussing circle for detection.

121' II'i apparatus for measuring the component wavelengths of radiationwithin the fluorescent X-ray spectrum of a material; a flexible crystalin the path of the radiation; a holder for said crystal comprising flxedapices bearing against a central portion of said crystal and movablecontact members bearing against end portions of said crystal for bendingsaid crystal about said apices to present a generally convex surface ofVariable curvature to the radiation, said holder being rotatably mountedabout an axis substantially coincident with said apices whereby saidcrystal may be rotated to monochromatize the component wavelengths alongspatially distributed paths intersecting a focussing circle; detectingmeans movable along said fccussing circle for receiving the componentwavelengths; and motion translating means for rotating said holder andcrystal. moving said contact members and moving said detecting means insynchronism whereby each of the component wavelengths may be focussedsuccessively along said focussing circle for detection.

13. An attachment for use with X-ray diffraction goniometer apparatus toobtain the fluorescent X-ray spectrum of a material comprising arotatably mountable crystal holder including fixed apices coincidentwith the axis of rotation of said holder and spaced movable contactmembers between which a flexible crystal may be supported to bear at itscenter against said apices, and means attached to said movable contactmembers adapted to bear against a stationary cam to move said contactmembers as said holder is rotated about its axis.

14. An attachment for use with X-ray diffraction goniometer apparatus toobtain the fluorescent X-ray spectrum of a material comprising arotatably mountable crystal holder including fixed apices coincidentwith the axis of rotation of said holder and spaced simultaneouslymovable contact members between which a flexible crystal may besupported to bear at its center against said apices, the direction ofmovement of said contact members being substantially perpendicular to aplane including said apices and the contact portions of said contactmembers, and means attached to said movable contact members adapted tobear against a stationary cam to move said contact members as saidholder is rotated about its axis.

15. An attachment for use with X-ray diffraction goniometer apparatushaving a source of X rays, detecting means movable along a fooussingcircle, a rotatably mounted specimen holder, and motion translatingmeans for rotating the specimen holder and moving said detecting meansin synchronism; said attachment comprising a crystal holder adapted toreplace the specimen holder including fixed apices coincident with itsaxis of rotation and spaced movable contact members between which aflexible crystal may be supported to hear at its center against saidapices, a stationary cam adapted to be positioned adjacent said crystalholder, and means attached to said movable contact members and adaptedto 12 bear against said stationary cam to move said Number Name Datecontact members as said crystal holder is ro- (r 2,452,045 7 FriedmanOct. 26, 1%8 tated. 2,474,835 'Friedman July 5, 1949 7 DAVID HARKER' VOTHER REFERENCES REFERENCES CITED 5 Philips Technical Review, v01. 10,No. 1, pp. 9 a 1-36, July 1948. 7

y: i l ggfii fi q are of m the Focusing X-ray monochromators, by C. S.Smith, Review Scientific Instruments, June 1941, UNITED STATES PATENTS10 DD. 312-314.

Number Name Date 2,449,066 Friedman Sept. 14, 1948 Certificate ofCorrection Patent No. 2,540,821 February 6, 1951 DAVID HARKER It ishereby certified that error appears in the printed specification of theabove numbered patent requiring correction as follows:

Column 6, line 35, for distance L /2R read distance L /2R and that thesaid Letters Patent should be read as corrected above, so that the samemay conform to the record of the casein the Patent Office.

Signed and sealed this 19th day of June, A. D. 1951.

THOMAS r. MURPHY,

Assistant Oommz'ssz'oner of Patents.

